2 Answers
2

Check the image source location to ensure that permissions inheritance
is not broken from the main site. Then check to make sure the images
therein do not have any custom permissions or broken permission
inheritance.

It may be possible the individual user you had granted access to the
site already had read or higher permissions from some other
permissions that were already established.

Where are the images stored? If they are in a different site, users will need access to them (i.e. permission to view the site and/or library). If they are in the same site, verify the library holding then inherits pernmissions or that your users have access to them.